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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On 10/9/26 16:56, sashiko-bot@kernel.org wrote: >> -static void vfio_device_file_set_kvm(struct file *file, struct kvm *kvm) >> +static void vfio_device_file_set_kvm(struct file *file, struct file *kvm) >> { >> struct vfio_device_file *df = file->private_data; >> + struct file *old; >> + >> + if (kvm) >> + get_file(kvm); > > [Severity: Critical] > Does taking a strong reference to the KVM file here create a circular > reference cycle? > > When userspace adds a VFIO device to a KVM VM, kvm_vfio_file_add() takes a > reference to the VFIO file. If vfio_device_file_set_kvm() then takes a > reference to the KVM file, both files hold references to each other. > > Because both file descriptors hold each other's references, their f_count > will never reach 0 when userspace closes the FDs, permanently leaking the > KVM VM and the VFIO structures. False positive, the reference to VFIO is in the KVM-VFIO device file and not in the KVM file descriptor itself. Using variable names, there is one reference from kvf to vfio and one from vfio to kvm. (Right? :)) Paolo > [ ... ] > >> diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c >> index 6cdc4e9a333a3..19548a4309426 100644 >> --- a/virt/kvm/vfio.c >> +++ b/virt/kvm/vfio.c >> @@ -144,6 +144,7 @@ static int kvm_vfio_file_add(struct kvm_device *dev, unsigned int fd) >> { >> struct kvm_vfio *kv = dev->private; >> struct kvm_vfio_file *kvf; >> + struct file *kvm_file __free(fput) = NULL; >> struct file *filp __free(fput) = NULL; >> >> filp = fget(fd); >> @@ -154,6 +155,10 @@ static int kvm_vfio_file_add(struct kvm_device *dev, unsigned int fd) >> if (!kvm_vfio_file_is_valid(filp)) >> return -EINVAL; >> >> + kvm_file = get_file_active(&dev->kvm->file); > > [Severity: High] > Does struct kvm contain a member named 'file'? Looking at > include/linux/kvm_host.h, it doesn't appear to exist, which would cause a > compilation error here. > >> + if (!kvm_file) >> + return -ENOENT; >> + >> guard(mutex)(&kv->lock); >> >> list_for_each_entry(kvf, &kv->file_list, node) { >> @@ -168,7 +173,7 @@ static int kvm_vfio_file_add(struct kvm_device *dev, unsigned int fd) >> kvf->file = get_file(filp); >> list_add_tail(&kvf->node, &kv->file_list); >> >> - kvm_vfio_file_set_kvm(kvf->file, dev->kvm); >> + kvm_vfio_file_set_kvm(kvf->file, kvm_file); > > [Severity: Critical] > This is where KVM takes its strong file reference on the VFIO device file, > which completes the reference cycle discussed above. >